Image projection devices, such as those used in lecture theatres etc, are well known. Typically, an image is written to an electrically addressed spatial light modulator (EASLM) and an optical arrangement projects a magnified image of the EASLM to a screen. Color images can be readily formed using three EASLM channels to project red, green and blue images to the screen.
Although conventional projection techniques are adequate to display television pictures and the like, the total number of pixels that can be displayed is limited to the number of EASLM pixels. Previously, images having a higher pixel count have been produced by using a number of separate projectors to build up sub-regions of an overall image. For example, the Department of Energy of the United State of America has developed a “high precision power wall” that uses a four-by-four array of projectors to produce a twenty-million color pixel display that operates at video rate.
The disadvantages of producing images by blending the outputs of a number of separate projector devices are numerous. For example, multi-projector systems typically produce noticeable artifacts at the boundaries of each projector's imaging area. Although attempts have been made to develop various software and hardware tools to minimize these unwanted artifacts, they have not proved particularly effective. Adjustment of the relative alignment of the projectors can also prove time consuming, and distortion introduced by the lenses will limit the degree of correction that is ultimately achievable. Furthermore, the color balance of projectors will vary with time and ensuring color uniformity across the image thus requires frequent re-calibration of the color output of the projectors.
WO00/40018 demonstrates how a high complexity image may be sequentially written to an optically addressed spatial light modulator (OASLM) using a so-called active tiling system. The high complexity two dimensional image stored by the OASLM can then be projected to a screen. Alternatively, a computer generated hologram may be written to the OASLM and used to provide a three dimensional (also termed volumetric) image. A number of computational methods used to calculate CGHs are described in Cameron et. al., “Computational challenges of emerging novel true 3D holographic displays”, paper 4109-23, presented at the SPIE conference on “Critical technologies for the future of computing”, August, San Diego, USA. Published in proc. SPIE vol. 4109.
Systems of the type described in WO00/40018 are particularly advantageous when massive pixel counts are required from a spatial light modulator in order to form volumetric images. However, the multiple active tiling projection channels coupled with the need for an OASLM make such devices complex and costly making them unsuitable for use as two-dimensional image projectors.